1. 15-744: Computer Networking
L-3 BGP

2. Next Lecture: Interdomain Routing
BGP
Assigned Reading
MIT BGP Class Notes
[Gao00] On Inferring Autonomous System Relationships in the Internet

3. Outline Need for hierarchical routing BGP Problems with BGP
ASes, Policies
BGP Attributes
BGP Path Selection
iBGP
Inferring AS relationships
Problems with BGP
Convergence
Sub optimal routing

4. Routing Hierarchies Flat routing doesn’t scale Key observation
Each node cannot be expected to have routes to every destination (or destination network)
Key observation
Need less information with increasing distance to destination
Two radically different approaches for routing
The area hierarchy
The landmark hierarchy

5. Areas Divide network into areas
Areas can have nested sub-areas
Constraint: no path between two sub-areas of an area can exit that area
Hierarchically address nodes in a network
Sequentially number top-level areas
Sub-areas of area are labeled relative to that area
Nodes are numbered relative to the smallest containing area

6. Routing Within area Outside area Can result in sub-optimal paths
Each node has routes to every other node
Outside area
Each node has routes for other top-level areas only
Inter-area packets are routed to nearest appropriate border router
Can result in sub-optimal paths

7. Path Sub-optimality start end 1 2 2.1 2.2 1.1 1.2 2.2.1 3
1.2.1
start
end
3.2.1 3
3 hop red path
vs.
2 hop green path
3.2
3.1

8. A Logical View of the Internet
National (Tier 1 ISP)
“Default-free” with global reachability info
Eg: AT & T, UUNET, Sprint
Regional (Tier 2 ISP)
Regional or country-wide
Eg: Pacific Bell
Local (Tier 3 ISP)
Eg: Telerama DSL
Tier 3
Tier 2
Tier 2
Customer
Provider
Tier 1
Tier 1
Tier 2

9. Landmark Routing: Basic Idea
Source wants to reach LM0[a], whose address is c.b.a:
Source can see LM2[c], so sends packet towards c
Entering LM1[b] area, first router diverts packet to b
Entering LM0[a] area, packet delivered to a
Not shortest path
Packet may not reach landmarks
LM0[a]
r1[b]
r0[a]
LM1[b]
LM2[c]
r2[c]
Network Node
Path
Landmark Radius

10. Landmark Routing: Example
d.d.a
d.d.b
d.d.c
d.d.e
d.d.d
d.d.f
d.i.k
d.i.g
d.d.j
d.i.i
d.i.w
d.i.u
d.d.k
d.d.l
d.n.h
d.n.x
d.n.n
d.n.o
d.n.p
d.n.q
d.n.t
d.n.s
d.n.r
d.i.v

11. Routing Table for Router g
Landmark
Level
Next hop
LM2[d]
LM0[e]
LM1[i]
LM0[k]
LM0[f] 2 1 f k
d.d.a
d.d.b
d.d.c
d.d.e
d.d.d
d.d.f
d.i.k
d.i.g
d.d.j
d.i.i
d.i.w
d.i.u
d.d.k
d.d.l
d.n.h
d.n.x
d.n.n
d.n.o
d.n.p
d.n.q
d.n.t
d.n.s
d.n.r
Router g
r0 = 2, r1 = 4, r2 = 8 hops
How to go from d.i.g to d.n.t? g-f-e-d-u-t
How does path length compare to shortest path? g-k-I-u-t
Router t

12. Outline Need for hierarchical routing BGP ASes, Policies
BGP Attributes
BGP Path Selection
iBGP
Inferring AS relationships

13. Autonomous Systems (ASes)
Autonomous Routing Domain
Glued together by a common administration, policies etc
Autonomous system – is a specific case of an ARD
ARD is a concept vs AS is an actual entity that participates in routing
Has an unique 16 bit ASN assigned to it and typically participates in inter-domain routing
Examples:
MIT: 3, CMU: 9
AT&T: 7018, 6341, 5074, …
UUNET: 701, 702, 284, 12199, …
Sprint: 1239, 1240, 6211, 6242, …
How do ASes interconnect to provide global connectivity
How does routing information get exchanged

14. Nontransit vs. Transit ASes
ISP 2
ISP 1
Nontransit AS
might be a corporate
or campus network.
Could be a “content
provider”
NET A
Traffic NEVER
flows from ISP 1
through NET A to ISP 2
(At least not intentionally!)
IP traffic

15. Customers and Providers
IP traffic
customer
Customer pays provider for access to the Internet

16. The Peering RelationshipC B
Peers provide transit between
their respective customers
Peers do not provide transit
between peers
Peers (often) do not exchange $$$
peer
customer
provider
traffic
allowed
traffic NOT
allowed

17. Peering Wars Peer Don’t Peer Reduces upstream transit costs
Can increase end-to-end performance
May be the only way to connect your customers to some part of the Internet (“Tier 1”)
You would rather have customers
Peers are usually your competition
Peering relationships may require periodic renegotiation
Peering struggles are by far the most
contentious issues in the ISP world!
Peering agreements are often confidential.

18. Routing in the Internet
Link state or distance vector?
No universal metric – policy decisions
Problems with distance-vector:
Bellman-Ford algorithm may not converge
Problems with link state:
Metric used by routers not the same – loops
LS database too large – entire Internet
May expose policies to other AS’s

19. Solution: Distance Vector with Path
Each routing update carries the entire path
Loops are detected as follows:
When AS gets route check if AS already in path
If yes, reject route
If no, add self and (possibly) advertise route further
Advantage:
Metrics are local - AS chooses path, protocol ensures no loops

20. BGP-4 BGP = Border Gateway Protocol Is a Policy-Based routing protocol
Is the EGP of today’s global Internet
Relatively simple protocol, but configuration is complex and the entire world can see, and be impacted by, your mistakes.
EGP –exterior gateway protocol , not to be confused with specific protocol used in earlier ARPANET etc.. Different revisions..current version is pretty new ! Add ons – functionality, addressing
1989 : BGP-1 [RFC 1105]
Replacement for EGP (1984, RFC 904)
1990 : BGP-2 [RFC 1163]
1991 : BGP-3 [RFC 1267]
1995 : BGP-4 [RFC 1771]
Support for Classless Interdomain Routing (CIDR)

21. BGP Operations (Simplified)
Establish session on
TCP port 179
AS1
BGP session
Exchange all
active routes
AS2
While connection
is ALIVE exchange
route UPDATE messages
Exchange incremental
updates

22. Interconnecting BGP Peers
BGP uses TCP to connect peers
Advantages:
Simplifies BGP
No need for periodic refresh - routes are valid until withdrawn, or the connection is lost
Incremental updates
Disadvantages
Congestion control on a routing protocol?
Inherits TCP vulnerabilities!
Poor interaction during high load

23. Four Types of BGP Messages
Open : Establish a peering session.
Keep Alive : Handshake at regular intervals.
Notification : Shuts down a peering session.
Update : Announcing new routes or withdrawing previously announced routes.
announcement =
prefix + attributes values

24. Policy with BGP BGP provides capability for enforcing various policies
Policies are not part of BGP: they are provided to BGP as configuration information
BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s
Import policy
What to do with routes learned from neighbors?
Selecting best path
Export policy
What routes to announce to neighbors?
Depends on relationship with neighbor

25. Examples of BGP Policies
A multi-homed AS refuses to act as transit
Limit path advertisement
A multi-homed AS can become transit for some AS’s
Only advertise paths to some AS’s
Eg: A Tier-2 provider multi-homed to Tier-1 providers
An AS can favor or disfavor certain AS’s for traffic transit from itself

26. Export Policy An AS exports only best paths to its neighbors
Guarantees that once the route is announced the AS is willing to transit traffic on that route
To Customers
Announce all routes learned from peers, providers and customers, and self-origin routes
To Providers
Announce routes learned from customers and self-origin routes
To Peers

27. Import Routes provider route customer route peer route ISP route From

28. Export Routes provider route peer route customer route ISP route To
From
provider To
peer To
peer To
customer To
customer
filters
block

29. BGP UPDATE Message List of withdrawn routes
Network layer reachability information
List of reachable prefixes
Path attributes
Origin
Path
Metrics
All prefixes advertised in message have same path attributes

30. Path Selection Criteria
Information based on path attributes
Attributes + external (policy) information
Examples:
Hop count
Policy considerations
Preference for AS
Presence or absence of certain AS
Path origin
Link dynamics

31. Important BGP Attributes
Local Preference
AS-Path
MED
Next hop

32. LOCAL PREF
Local (within an AS) mechanism to provide relative priority among BGP routers R5 R1
AS 200 R2
AS 100
AS 300 R3
Local Pref = 500
Local Pref =800 R4
I-BGP
AS 256

33. LOCAL PREF – Common Uses
Handle routes advertised to multi-homed transit customers
Should use direct connection (multihoming typically has a primary/backup arrangement)
Peering vs. transit
Prefer to use peering connection, why?
In general, customer > peer > provider
Use LOCAL PREF to ensure this

34. AS_PATH AS 200 AS 100 AS 300 AS 500 List of traversed AS’s
Useful for loop checking and for path-based route selection (length, regexp)
AS 200
AS 100
/16
/16
AS 300 / /
AS 500

35. Multi-Exit Discriminator (MED)
Hint to external neighbors about the preferred path into an AS
Non-transitive attribute
Different AS choose different scales
Used when two AS’s connect to each other in more than one place

36. MED
Typically used when two ASes peer at multiple locations
Hint to R1 to use R3 over R4 link
Cannot compare AS40’s values to AS30’s
MED = 50 R1 R2
AS 10
AS 40
MED = 120
MED = 200 R3 R4
AS 30

37. MED MED is typically used in provider/subscriber scenarios
It can lead to unfairness if used between ISP because it may force one ISP to carry more traffic:
ISP1 SF
ISP2 NY
ISP1 ignores MED from ISP2
ISP2 obeys MED from ISP1
ISP2 ends up carrying traffic most of the way

38. Route Selection Process
Highest Local Preference
Enforce relationships
Shortest ASPATH
Lowest MED
Traffic engineering
i-BGP < e-BGP
Lowest IGP cost
to BGP egress
Throw up hands and
break ties
Lowest router ID

39. Internal vs. External BGP
BGP can be used by R3 and R4 to learn routes
How do R1 and R2 learn routes?
Option 1: Inject routes in IGP
Only works for small routing tables
Option 2: Use I-BGP R1
E-BGP
AS1 R3 R4
AS2 R2

40. Internal BGP (I-BGP) Same messages as E-BGP
Different rules about re-advertising prefixes:
Prefix learned from E-BGP can be advertised to I-BGP neighbor and vice-versa, but
Prefix learned from one I-BGP neighbor cannot be advertised to another I-BGP neighbor
Reason: no AS PATH within the same AS and thus danger of looping.

41. Internal BGP (I-BGP) AS1 AS2 R3 can tell R1 and R2 prefixes from R4
R3 can tell R4 prefixes from R1 and R2
R3 cannot tell R2 prefixes from R1
R2 can only find these prefixes through a direct connection to R1
Result: I-BGP routers must be fully connected (via TCP)!
contrast with E-BGP sessions that map to physical links R1
E-BGP
AS1 R3 R4
AS2 R2
I-BGP

42. Each RR passes only best routes, no longer N2 scaling problem
Route Reflector
eBGP update RR
iBGP updates RR RR
Mesh does not scale
Each RR passes only best routes, no longer N2 scaling problem

43. Policy Impact Different relationships – Transit, Peering
Export policies  selective export
“Valley-free” routing
Number links as (+1, 0, -1) for customer-to-provider, peer and provider-to-customer
In any path should only see sequence of +1, followed by at most one 0, followed by sequence of -1

44. How to infer AS relationships?
Can we infer relationship from the AS graph
From routing information
From size of ASes /AS topology graph
From multiple views and route announcements
[Gao01]
Three-pass heuristic
Data from University of Oregon RouteViews
[SARK01]
Data from multiple vantage points

45. [Gao00] Basic Algorithm
Phase 1: Identify the degrees of the ASes from the tables
Phase 2: Annotate edges with “transit” relation
AS u transits traffic for AS v if it provides its provider/peer routes to v.
Phase 3: Identify P2C, C2P, Sibling edges
P2C  If and only if u transits for v, and v does not, Sibling otherwise
Peering relationship ?

46. How does Phase 2 work? Notion of Valley free routing
Each AS path can be
Uphill
Downhill
Uphill – Downhill
Uphill – P2P
P2P -- Downhill
Uphill – P2P – Downhill
How to identify Uphill/Downhill
Heuristic: Identify the highest degree AS to be the end of the uphill path (path starts from source)

47. Next Lecture: Congestion Control
Monday: optional review of transport and above
Congestion Control
Assigned Reading
[Chiu & Jain] Analysis of Increase and Decrease Algorithms for Congestion Avoidance in Computer Networks
[Jacobson and Karels] Congestion Avoidance and Control

48. Safety: No Persistent Oscillation
1 3 0
1 0 1 2 3
Jargon!! Remind us about filtering, ranking, and the high-level intuitive description of the property
Start off describing what AS 1 does
Put on varadhan et al.
2 1 0
2 0
3 2 0
3 0
Varadhan, Govindan, & Estrin, “Persistent Route Oscillations in Interdomain Routing”, 1996

49. Main Idea of Optional Paper
Permit only two business arrangements
Customer-provider
Peering
Constrain both filtering and ranking based on these arrangements to guarantee safety
Surprising result: these arrangements correspond to today’s (common) behavior
Gao & Rexford, “Stable Internet Routing without Global Coordination”, IEEE/ACM ToN, 2001